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What should I do if I want to change the header on my site but the program won't let me--it says that it can't "modify header information?"
This is commonly known as the "whitespace problem".
The error message typically looks something like
The problem is that many editors seem to add additional blanks (spaces) and/or empty lines at the end of a file when you edit it or may have been caused by your FTP or text editor software.
Warning: Cannot modify header information - headers already sent by..... (line numbers and file names may vary)